Many currently produced vehicles, including but not necessarily limited to light duty trucks and passenger vehicles, are equipped with electronically controlled transmissions. These transmissions may be automatic or manual transmissions. Many of these transmissions contain an overdrive feature which is electronically controlled. This electronic control may be logically implemented as a CMOS chip, manually by means of a switch, or a combination of both. CMOS control may be implemented internally in the transmission itself, or by means of power train control module (PCM), transmission control module (TCM), or other external logic module. The most commonly occurring implementation of this general design strategy is that where the transmission is biased toward the overdrive mode of operation with each ignition run cycle by means of a PCM. It should be noted that the terms "PCM" and "TCM" as used herein include, but are not necessarily limited to, their normal SAE J1930 meanings.
While the previously discussed normally biased mode of operation is preferred for many applications, including passenger service and the transport of light loads in relatively flat terrain, there are some vehicle applications where the preferred mode of transmission operation is the non-overdrive, or standard drive, mode of operation. Examples of some such vehicle applications include truck fleets, taxi cab fleets, limousine fleets, motor home drivers, and tow truck drivers who often run their vehicles at close to the vehicle's gross weight capacity and/or in hilly areas, and vehicles involved in towing. Where the overdrive mode of transmission operation is selected and the vehicle is used in these exemplar applications, several negative concomitants ensue: first, brake wear is increased because the engine braking feature is disconnected with overdrive operation; wear on the transmission's torque converter as well as the transmission's bands and clutches are increased due to constant shifting in and out of overdrive ("hunting") when driving in hilly terrain; finally, for both of the foregoing reasons, vehicle safety may be negatively impacted.
The previously discussed problem is often exacerbated when the vehicles are operated as part of a fleet, or rental operation. In this situation the vehicles' owners must rely on their driver-employees, or vehicle renters, to manually de-select overdrive mode when hauling heavy loads or driving over hilly terrain. Operators of these fleet vehicles often forget to do so and do not, therefore, manually disengage overdrive. Accordingly, the vehicle owner suffers increased brake and transmission maintenance costs.
In order to overcome the previously discussed problem,mechanical "overdrive halvers" have been developed as an after-market device. After-market dealers mount these devices between the vehicle's transmission and rear differential thereby mechanically reducing the effect of the transmission overdrive. It should be noted that these devices do not actually disengage the overdrive function and therefore do not provide a true non-overdrive mode of operation. Furthermore, these mechanical devices are relatively expensive to install.
What is needed then is a simple, effective method for inverting the function of the overdrive select feature of electronically-controlled transmissions. The method, and an apparatus to practice that method, should preferably be durable, of moderate cost, and retain full functionality of the vehicle's overdrive system.
Further, the method should automatically invert the non-biased mode of operation each time the vehicle ignition is set to "on". In most cases, this will have the effect of selecting the non-overdrive mode of operation each time the vehicle ignition is switched on.
In a normal cycle, the vehicle key (and hence the ignition switch) is cycled from off to the run position, momentarily to the start position and back to the run position. During the development of the present invention, one additional problem became clear: some early models of an apparatus developed to perform the method taught and claimed herein reset themselves with each engine crank cycle, as opposed to each ignition run cycle. This is important where more than one effort must be made to start the engine during a given ignition cycle. This is often the case in diesel engines, and occasionally occurs in other types of internal combustion engines as well. What is further needed then, is that the method for inverting the function of the overdrive select feature possess a "multiple crank" capability, which capability ensures that only one inverter signal be transmitted to the transmission's overdrive control for each ignition run cycle, even if several efforts, or start cycles, must be made to start the engine within the run cycle. This would preclude the inadvertent biasing of the transmission back into the overdrive mode responsive to a second or subsequent start attempt.
Finally, the functionality of the overdrive select button and driver's overdrive indicator, if fitted, should be retained: the vehicle operator should retain the option of manually engaging overdrive when it is desirous to do so, and should continue to have indication of the overdrive mode selected. When engaged, normal operation of the vehicle's overdrive mode should not in any way be impinged.